1. Related Application
U.S. patent application Ser. No. 07/587,190 filed Sept. 24, 1990 entitled "PMG-based Position Sensor and Synchronous Drive Incorporating Same".
2. Field of the Invention
This invention relates to ac drives incorporating a power converter which generates a multi-phase stator current phase locked to rotor position at a phase angle which is adjustable to produce optimum torque as a function of speed.
3. Background Information
Commonly owned U.S. patent application Ser. No. 07/587,190 filed on Sept. 24, 1990 discloses an ac drive in which a power electronic converter provides the multi-phase stator current for a synchronous motor. The multi-phase stator current is fed back negatively to the input of the converter which therefore acts as a current source. The converter is controlled by a multi-phase current reference voltage which is phase locked by a high resolution phase locked looped to a rotor position signal generated by a permanent magnet generator mounted on the motor shaft. This arrangement provides a rugged ac drive suitable for use in difficult applications subject to hostile environments such as aircraft starter/generators. In such an ac drive, the multi-phase stator current is phase locked at a predetermined phase angle to rotor position, typically selected to produce maximum torque. However, for a machine with fixed rotor flux, the maximum speed at which full torque can be generated is limited, and torque and power drop off rapidly at that point.
There is a need, therefore, to provide an improved ac drive incorporating a controlled current power converter in which the maximum speed obtainable can be extended beyond that presently available in such drives.
More broadly, there is a need for an ac drive incorporating a controlled current power converter in which the torque can be controlled as a function of speed.
More particularly, there is need for such an improved ac drive in which the phase angle between the stator current and the rotor position can be controlled to control torque.